The present invention relates to a color shutter for field-sequentially separating a white light into RGB components, and a display apparatus for displaying a color image by field-sequentially mixing the color components.
A system for displaying a color image includes a spatial division display for displaying an RGB image by dividing the RGB image into the RGB components for pixels and a field-sequential additive color mixing display in which an RGB image is displayed with time and an RGB color filter is switched in synchronization with the image.
The field-sequential additive color mixing display is superior to the spatial division display in fineness because the pixel need not be divided into RGB components in the field-sequential additive color mixing display. In the field-sequential additive color mixing display, the method of rotating a disc-like filter divided into three color regions of RGB in synchronization with each RGB image display is most widely known to the art.
A method of switching the displayed color without using a mechanical rotation mechanism is disclosed in, for example, U.S. Pat. No. 5,387,920 to Bos et al. Specifically, proposed is a so-called xe2x80x9cliquid crystal color shutter systemxe2x80x9d in which a color polarizer is arranged on each of the front surface and the rear surface of two liquid crystal cells, and the polarizing plane of the light is controlled by the on/off switching of the liquid crystal cells so as to select the wavelength of light absorbed by the polarizer and, thus, to achieve an RGB display.
In the liquid crystal color shutter disclosed in this prior art, a plurality of color polarizers differing from each other in the color phase are arranged on an optical path such that the absorption axes are rendered perpendicular to each other. For example, a yellow color polarizer, which transmits green and red lights and absorbs a wavelength region of blue, and a blue color polarizer, which absorbs a wavelength region of yellow, are arranged such that the absorption axes of these two polarizers are rendered perpendicular to each other. Similarly, a red color polarizer and a cyan color polarizer are arranged such that the absorption axes of these two polarizers are rendered perpendicular to each other. Further, a liquid crystal cell is arranged between these two sets of color polarizers, and an achromatic polarizer, which is a linear full wavelength polarizer region, and a liquid crystal cell are added so as to select the axis of polarization of the incident light or leaving light.
It should be noted that a red display can be achieved by using a polarized light transmitting through the absorption axes of the yellow color polarizer and the red color polarizer. Also, a green display can be achieved by the combination of the yellow color polarizer and the cyan color polarizer. Further, a blue display can be achieved by the combination of the blue color polarizer and the cyan color polarizer.
The liquid crystal color shutter is advantageous in that a mechanical operation is not involved therein, and its space saving because the area of the display screen can be made equal to the area of the color shutter.
In the liquid crystal color shutter, a colorant-based color polarizer prepared by impregnating a PVA (polyvinyl alcohol) substrate with a dichroic colorant, followed by applying an orienting treatment by stretching to the impregnated PVA substrate is used as the color polarizer. As shown in FIG. 1, the colorant-based color polarizer is a partial polarizer capable of absorbing a specified wavelength of a polarized light having a polarizing plane in the direction of the absorption axis.
The liquid crystal color shutter using the particular colorant-based color polarizer is defective in that the transmittance is markedly low. For example, the absorption axis transmitting characteristics of the colorant-based color polarizer are shown in FIG. 6 of a literature xe2x80x9cProceedings of the SIDxe2x80x9d Vol. 26/2 (1985), 157-161.
As apparent from FIG. 6 of the literature quoted above, curves of the transmittance characteristics are sharp and the dichroic ratio is sufficient in the red polarizer and the yellow polarizer. However, the characteristics of the blue polarizer and the cyan polarizer are markedly inferior. Therefore, where a liquid crystal color shutter is formed by using these colorant-series color polarizers, the transmittance is markedly lowered.
Under the circumstances, a PRS (Polarizer Retarder Stack) is proposed in recent years by Sharp et al. as a color polarizer performing the function similar to that performed by a dichroic color polarizer and used in a liquid crystal color shutter in place of the dichroic color polarizer (U.S. Pat. No. 5,751,384). The PRS is formed by laminating a plurality of birefringent retardation films (i.e., at least about 5 films) on an achromatic polarizer in a phase axis direction of a predetermined angle.
By setting the retardation and the phase axis direction of the birefringent retardation film appropriately in the PRS, it is possible to allow the white light incident on the side of the achromatic polarizer to be emitted from the polarizer at different angles relative to the optical axis of the achromatic polarizer depending on the wavelength region of the light, as shown in FIG. 2. For example, the light having the wavelength region of blue is emitted at 0xc2x0 and the light having wavelength regions of green and red (yellow) is emitted at 90xc2x0 relative to the optical axis of the achromatic polarizer. It follows that this example is equal to the case where dichroic colorant polarizers of blue and yellow are arranged such that the absorption axes of these two polarizers are perpendicular to each other. The PRS does not include an absorption medium other than the absorption axis of the achromatic polarizer and, thus, has a high transmittance, compared with the dichroic colorant polarizer.
The liquid crystal color shutter employs the system that the transmitting color is switched by controlling the polarizing plane of the incident light. Therefore, where an unpolarized natural light is assumed to be the incident light, one polarized component is absorbed during conversion from the unpolarized light into a polarized light. Thus, the liquid crystal color shutter is essentially lower in its transmittance than the color filter. Naturally, it is important to improve the transmittance of the liquid crystal color shutter.
The optical characteristics of the color polarizer constituting the liquid crystal shutter greatly affects the transmittance of the liquid crystal color shutter. When it comes to the two color polarizers of the dichroic colorant polarizer and the PRS, the PRS system is advantageous in transmittance because the absorbing member is not included in the members other than the achromatic polarizer. On the other hand, in the PRS system, the incident light is separated into mutually complementary colors such as blue/yellow or cyan/red in the axes of the polarized light perpendicular to each other. It follows that it is impossible to cut the undesired light, with the result that the component of the intermediate wavelength region in the boundary region between blue and green and between green and red is allowed to be contained in any of the color display of the RGB displays. Such being the situation, it is difficult to improve the color purity in all of RGB colors.
As an example specifically showing the above-noted problem, the construction of the conventional liquid crystal color shutter using PRS, the transmittance characteristics at each PRS, and the RGB color reproducing region in the CIE1976UCS chromaticity diagram are shown in FIGS. 3 to 9.
Specifically, FIG. 3 shows as an example the construction of a LCCS (liquid crystal color shutter) using PRS. Polarizing rotators 103, 104 consisting of liquid crystal cells are inserted between achromatic polarizers 105 and 106 and between the polarizers 106 and 107, respectively. By controlling the voltage applied to each of these polarizing rotators 103 and 104, it is possible to select in a binary fashion the transmission/90xc2x0 rotation of the polarizer, making it possible to give four kinds of polarized states to the incident light. Further, birefringent retardation films 108 and 109 are inserted such that the achromatic polarizer 105 and the birefringent retardation film 108 constitute a PRS structure. Likewise, the achromatic polarizer 106 and the birefringent retardation film 109 constitute another PRS structure.
FIG. 4 shows the relationship between the transmittance and the wavelength with respect to the polarized component of the light passing through the achromatic polarizer 105 and the birefringent retardation film layer 108, said polarized component having axes of polarization in the directions of a transmission axix and an absorption axis of the achromatic polarizer 105. The birefringent retardation film 108 is of a five-layer structure, each layer having a retardation value of 600 nm and the directions of the fast axes of these five layers being arranged at 45xc2x0/xe2x88x9215xc2x0/xe2x88x9215xc2x0/10xc2x0/10xc2x0 relative to the transmission axis of the achromatic polarizer 105. As apparent from FIG. 4, a color polarizer of yellow/blue is formed in this PRS structure.
On the other hand, FIG. 5 shows the relationship between the transmittance and the wavelength with respect to the polarized light component of the light passing through the achromatic polarizer 106 and the birefringent retardation film layer 109, said polarized light component having axes of polarization in the directions of a transmission axis and an absorption axis of the achromatic polarizer. The birefringent retardation film layer 109 is of a six-layer structure, each layer having a retardation value of 643 nm, and the directions of the fast axes being arranged at 8.3xc2x0/18xc2x0/18xc2x0/xe2x88x923.7xc2x0/xe2x88x9245xc2x0/xe2x88x9278xc2x0 relative to the transmission axis of the achromatic polarizer 106. As apparent from FIG. 5, a red/cyan color polarizer is formed in this PRS structure.
By the combination of these two kinds of the PRS structures and the achromatic polarizer 107, the RGB transmission characteristics obtained by the voltage control of the liquid crystal cells 103 and 104 are represented by 3001 (blue), 3002 (green) and 3003 (red) as shown in FIG. 6.
Suppose an image display apparatus of a field-sequential color mixing display is formed by arranging the liquid crystal color shutter described above on the front surface of a monochromatic CRT. In this case, the color reproducing region calculated in view of the emission spectrum of a standard phosphor P22 for a TV used as a light source is as shown in FIG. 7. Incidentally, the emission spectrum of P22 is denoted in FIG. 6 by a reference numeral 704.
In the CIE1976UCS chromaticity diagram shown in FIG. 7, the RGB color reproducing region in the ordinary CRT, in which the RGB display was performed by the spatial color mixing display, is represented by the reference numeral 801. On the other hand, the RGB color reproducing region by the conventional construction as shown in FIG. 3 is represented by reference numeral 3101. As apparent from FIG. 7, the RGB color reproducing region 3101 in the conventional structure as shown in FIG. 3 is insufficient in the chroma of red and blue, compared with the RGB color reproducing region 801 in the ordinary CRT.
It is possible to shift the transmittance characteristics shown in FIGS. 4 and 5 by changing the retardation values of the two kinds of PRS structures. However, if the chroma of each of red and blue is improved, the chroma of green is lowered. In other words, the color purity of each of RGB bears a trade-off relationship.
Even where the phosphor for CRT is changed into a three wavelength type as another example, the chroma of blue is certainly improved if the RGB color reproducing properties are similarly calculated on the assumption of the typical P45 phosphor (FIG. 28, 904). However, the chroma of red becomes greatly deficient as denoted by the reference numeral 3301 in FIG. 9.
On the other hand, in order to improve the color purity by absorbing the light component having an intermediate wavelength region, Sharp et al. have proposed a system in which a pre-filter, i.e., an achromatic polarizer and a plurality of birefringent retardation films, is added to a light shutter based on the PRS system (FIGS. 38 and 39 of U.S. Pat. No. 5,929,946).
In this system, however, it is difficult to control independently the absorption wavelength region and the absorption profile in the intermediate wavelength regions between blue and green and between green and red. Also, the addition of the achromatic polarizer and the birefringent retardation films makes the construction and the manufacturing process complex and causes reduction in the transmittance because of the transmission loss of the films.
As described above, it is necessary to improve the color purity of the RGB displayed colors while improving the transmittance in the liquid crystal color shutter. In the constructions proposed to date, however, there were some merits and some demerits simultaneously, making it difficult to satisfy these two requirements simultaneously.
An object of the present invention is to provide a color shutter that permits improving the color purity of each of RGB displayed colors while improving the transmittance.
Another object of the present invention is to provide a color image display apparatus provided with a color shutter of the present invention.
According to a first aspect of the present invention, there is provided a color shutter, comprising first, second and third polarizers polarizing the incident light over the entire region of the visible wavelengths and arranged in the order mentioned as viewed from the side of the incident light; first and second retarders each having a plurality of birefringent layers; first and second polarizing rotators changing the angle of rotation of the polarizing plane by application of a voltage over the entire region of the visible wavelengths of the incident light; and at least one absorption type partial polarizer converting the incident light into a polarized light by absorbing a part of the wavelength regions of the incident light, wherein the first retarder is arranged between the first polarizer and the second polarizer; the second retarder is arranged between the second polarizer and the third polarizer; the first polarizing rotator is arranged between the first polarizer and the second polarizer; the second polarizing rotator is arranged between the second polarizer and the third polarizer; at least one of the first polarizing rotator and the second polarizing rotator is arranged in a manner to be sandwiched between the second polarizer and the first or second retarder; the absorption type partial polarizer is arranged between the first polarizing rotator and the first retarder and/or between the second polarizing rotator and the second retarder; and the transmitted light can be changed into the three primary colors of RGB by selecting the voltage applied to the first and second polarizing rotators.
According to a second aspect of the present invention, there is provided a color image display apparatus, comprising an image display mechanism displaying a monochromatic two dimensional image, and the color shutter of the present invention arranged on the front surface of the display screen of the image display mechanism, wherein the image display mechanism sequentially displays a monochromatic image for the three primary colors of RGB, and the transmitted color of the color shutter can be switched in synchronization with the image display.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.